This invention relates to a fluid catalytic cracking process using a distillation residual oil, its solventdeasphalted product, or its hydrodesulfurized product as a starting oil.
Fluid catalytic cracking, known simply as FCC in the art, is a process whereby a petroleum hydrocarbon as a raw material is cracked by contact with a fluidized catalyst to obtain a product the major portion of which consists of gasoline, liquefied petroleum gases, alkylating materials and middle distillates.
Usually, a gas oil is used as a starting oil in fluid catalytic cracking. The gas oil, as used herein, denotes a heavy gas oil from an atmospheric distillation unit, distillates such as vacuum gas oils from a vacuum distillation unit, or the hydrogenation products thereof. It has a boiling point in the range of 220.degree. C. to 600.degree. C. and a specific gravity of about 0.8 to 1.0.
In recent years, while crude oils produced have tended to contain heavy components, there has been a relatively increasing demand for hydrocarbon oils containing fractions having a boiling point lower than the gas oil in view of the environmental problem or the ease of utilization. Hence, use of the gas oil alone as a raw material of FCC has given rise to a problem from the standpoint of raw material resources. In order to save energy, too, effective utilization of residual oils has become an important problem. Attempts have therefore been made to use residual oils as a raw material for FCC.
In the FCC of a residual oil, a particularly well-known phenomenon is the deposition of nickel, vanadium, iron and copper contained in the starting oil on the catalyst. Usually, crude oils contain 5 to 500 ppm of nickel, 5 to 1500 ppm of vanadium, 1 to 100 ppm of iron, and 0.1 to 10 ppm of copper. Since the starting oil, on contact with transporting, storing and processing apparatuses, tends to dissolve the iron of the apparatuses, the actual content of iron in the starting oil greatly exceeds the above-mentioned value. Furthermore, since these metals tend to remain in the bottom during the distillation, the residual oil contains these metals in amounts 2 to 4 times as large as those of the starting oil. In some cases, the residual oils may contain as high as 1000 to 2000 ppm of vanadium.
These metals usually exist as organic metal compounds including porphyrin-like compounds. These compounds are decomposed on contact with the catalyst at high temperatures, and the metals deposit on the catalyst. The deposited metals reduce both the activity and the selectivity of the catalyst. These metals have hydrogenating-dehydrogenating activity, and under the reaction conditions of FCC, accelerate dehydrogenation of hydrocarbons. As a result, the amounts of undesirable hydrogen gas and coke increase, and the yields of the desired LPG, gasoline, kerosene and gas oils decrease.
Deposition of metals which exert an adverse effect on the reaction is not so important a problem in the FCC of gas oils. For the gas oils contain small amounts of these metals, and the amounts of metals deposited on the catalyst are generally small. Moreover, the amount of the catalyst required to be exchanged can be small. In fact, in the FCC of a gas oil, adverse effects of metal deposition on the catalyst can be avoided by simply supplying a fresh catalyst in an amount corresponding to that of the catalyst which naturally is carried over from the apparatus.
Since, however, the amounts of metals deposited are very large in the FCC of residual oils having a high metal content, a special means is necessary in order to retain the activity and selectivity of the catalyst. It is the usual practive to maintain the activity of the catalyst at a certain fixed level by periodically or steadily withdrawing a part of the catalyst to exchange it with a fresh catalyst or a reactivated catalyst (for example, reactivated by an ion exchange method or an oxidation-reduction method). However, the amount of the catalyst to be withdrawn should be very large, and such a means is economically very disadvantageous. Accordingly, the problem of metal deposition on the catalyst is especially serious in the FCC of residual oils having a high metal content.
It is this problem which the present inventors have attempted to solve.